Rusty tussock moth caterpillar

Orgyia antiqua

I am indebted to the Internet for the following.

The larvae hatch early in the spring, as soon as foliage starts to appear They are easily recognized by their horn-like tufts of hair-like setae. There are 4 toothbrush-like tufts along the back, and hair pencils that project from the sides at the front and at the back. The body is dark grey to black, and there are red tubercles along the sides and back. They have defensive glands at the back, and wipe their setae against them to charge them with toxins They grow to about 30-40 mm, females being considerably larger than males.

In case you don't know which is the head, the excreta has been included as a reference point.

In large it is even more scary